Method of polarizing ceramic transducers



July 28, 1953 A. w. WILLIAMS 2,645,510

METHOD OF POLARIZING CERAMIC TRANSDUCERS Filed April 27, 1951 2 Sheets-Sheet l INVENTOR. ALF-RED L. W. WILLIAMS A. L. w. WILLIAMS METHOD OF POLARIZUING CERAMISJ TRANSDUCERS Filed April 27, 1951 July 28 1953 ,2 Sheets-Sheet 2 INVENTOR. ALFRED L .W. WILLIAMS Patented July 28, 1953 METHOD OF POLARIZING CERAMIC TRANSDUCERS Alfred L. W. Williams, Cleveland Heights, Ohio, assignor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application April 27, 1951, Serial No. 223,389

9 Claims. 1 This invention relates generally to a method of polarizing a ceramic transducer and relates specifically to a method which is of particular utility in cases where it is difficult or impossible to polarize the ceramic material by the usual method of applying a voltage between two electrodes on opposite faces of the ceramic material. In recent years, ceramic electromechanical transducers have come into an extensive amount of use. Such transducers are generally comprised of a fired ceramic material which is comprised mainly of barium titanate, although in some cases the material may have added thereto a few Weight percent of some metal oxide in order to provide some particular operating character- 'sound' energy into a liquid medium, devices for producing mechanical energy from sound energy in a liquid medium, etc. Several such transducers are described and claimed in United States Letters Patent No. 2,486,560 granted on Novemher 1, 1949, on an application of Robert B. Gray. filed on September 20, 1946.

While transducers of the type here under consideration have found some use in devices where a continuous unidirectional bias voltage is applied while the transducers are in operation, it is the general practice to cause the material of such transducers to have a remanent polarization which renders the provision of the continuous unidirectional bias voltage during the operation'of the device unnecessary. Materials of the type here under consideration are caused to have a remanent polarization by subjecting the mate- 'rial to the action of a high-intensity electric field at some time during the manufacturing process of the device. This polarizing of the material is usually done after the material has been fired and after suitable electrodes have been applied to the surfaces of the material. However, the voltages which are required for polarizing devices in this manner are sometimes very high. It is usually necessary or desirable to apply voltages of the order of 75,000 volts per inch to the material where the polarizing is done at ambient room temperatures, although it is possible to reduce this voltage gradient requirement by about half where the polarizing is done near, and preferably just below, the Curie point of the material being polarized. The Curie point of barium titanate is around 120 centigrade. Where thick pieces of ceramic transducer material are to be polarized, the difficulties encountered are very much increased for several reasons. In the first place, the breakdown vcltage of most dielectric materials, including air, does not increase linear ly in accordance with the length of the path over which the voltage is applied. Specifically, a given voltage gradient across a thick piece of barium titanate transducer material is much more likely to ionize the surrounding air and cause a voltage breakdown across the surface of the material being polarized than is the same voltage gradient in the case where a very thin piece of ceramic material is being polarized in air.

The dielectric strengths of some fluid materials, such as some oils, are much higher than that of air. In order to reduce the difficulties mentioned above to the maximum possible extent, it has been the practice in many cases in the past, where difiicult polarizing requirements were encountered, to polarize the material in oil or parafiin and at a temperature just below the Curie point of the material. In such cases, it has been the general practice to allow the transducer material to cool to a considerable extent, and perhaps to room temperature, with the polarizing voltage being maintained between electrodes on the material being polarized.

Also, where high polarizing voltages are used in the general manner described above, it is frequently found that the surface conditions of the material being polarized are such as to cause a voltage breakdown between the electrodes. Such surfaces are frequently lapped or ground during the manufacturing processes and have thereon foreign material which tends to encourage the voltage breakdown. It has been found that these difiiculties can be obviated to a large extent if the material is thoroughly washed with soap and water before the polarizing operation is carried out.

From the above it will be seen that considerable difificulties are frequently encountered in polarizing ceramic transducer material. It would be desirable, therefore, to provide a method of polarizing such ceramic materal which allows the application of very high voltage gradients to the material but which does not require an unduly high polarizing voltage and which methods still can be utilized to efiectuate polarizaticn over a rather long path in the ceramic material. Applicant, by his present invention, has provided such a polarizing method.

Also, in some cases it is desirable to polarize ceramic transducer material in such a direction that it is inconvenient, and sometimes impossible, to provide fixed electrodes upon the ceramic material in order that the desired polarization may be effected by the application of a voltage between theelectrodes. Thus, it is well known that a remanent polarization in one direction in a ceramic plate with a signal field thereafter applied at right angles thereto will provide a shear action in the ceramic plate. In polarizing such material, it is frequently the custom to apply thereto electrodes to be used only for polarizing, these electrodes being thereafter removed in order that other electrodes can be applied to the material for use in applying the signal voltages in the required direction. In many cases it is not possible to have both the polarizing electrodes and the electrodes for receiving the applied signal voltages on the material at the same time because one set of electrodes tends to short out the voltage applied to the other set of electrodes. Applicant has provided a method by which it is possible to polarize rather long pieces of titanate material in the length direction without the necessity of applying electrodes to the end faces of the material for this purpose.

Also, it is sometimes desirable to provide a circumferential polarization in a right circular cylinder. It is not possible to effectuate such a polarization by the normal procedure of applying two electrodes to opposite faces of the material. Applicant has provided a polarizing procedure which can be effectively used to provide this circumferential polarization.

It is an object of the invention, therefore, to provide an improved method of polarizing ceramic transducer material.

It is an object of the invention to provide a method of polarizing ceramic transducer material which obviates the necessity for providing fixed electrodes on the surface of the material for use during the polarization process.

It is still another object of the invention to provide a method of polarizing relatively long strips or sheets of ceramic transducer material.

It is still a further object of the invention to provide a method of polarizing certain ceramic transducer bodies with a direction of polarization which is difiicult or impossible to provide by other methods.

In accordance with the invention, the method of polarizing in a predetermined direction a fired ceramic transducer material which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprises, applying a unidirectional polarizing voltage between a first two points spaced in the above-mentioned direction on a surface of the material for an appreciable period of time and of sufiicient magnitude as to eifect a polarization in the material between the two points. The method also comprises the step of subsequently applying a unidirectional polarizing voltage between a second two points on the surface of the material which are spaced from the first two points in the above-mentioned direction with oneof the second two points being between the first two points and the other of the second two points being spaced from said first two points in the above-mentioned predetermined direction.

In accordance with another feature of the invention, the method of polarizing in a predetermined direction a fired ceramic transducer material which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprises, applying a unidirectional polarizing voltage between a first two points spaced in the direction of desired polarization on a surface of the material for an appreciable period of time and of suflicient magnitude as to effect a polarization in the materialbetween these two points. This method also includes the subsequent step of effectively moving, in the desired direction of polarization on the surface of the material, the points of application of the polarizing voltage which is applied to the material.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

Fig. 1 of the drawings is used to illustrate a preferred method of the invention as utilized to polarize a relatively thin plate of ceramic transducer material in the length direction of the material; Fig. 2 is used to illustrate a modification of the Fig. 1 embodiment; Fig. 3 is used to illustrate the method generally disclosed by the Fig. 2 arrangement as applied to a cylindrical ceramic transducer body; while Fig. 4 is used to illustrate a modification of the Fig. 3 embodiment which is particularly useful in efiectuating a polarization in bodies which are relatively thick in a direction normal to the direction of desired polarization.

Referring now to Fig. 1 of the drawings, there is shown a plate [0 of ceramic transducer material which is to be polarized in the length direction. Two conductive bars II and I2 are provided so that a polarizing voltage may be provided between them and so that the two bars may be moved as a unit in the length direction along the surface of the plate H). In order to enable the bars to be moved as a unit, two insulating spacers l4 and [5 are provided, these being fixedly secured to the bars. Thus screws I6 and I? are effective to fasten the insulating spacer l4 to the bars I l and [2, respectively, and fastening members IS and 2B, which also serve as electrical terminals for applying a voltage between the rods I l and I2, are effective to fasten the insulating spacer to the rods H and 12, respectively. A suitable source of polarizing voltage is applied to the conductive members [9 and from a source of D. C. voltage, indicated by the reference numeral 22.

In considering the operation of the arrangement just described, it will be seen that the application of a voltage between the bars H and I2 causes a field pattern in the material somewhat as represented by the dotted lines 23. The direction of this field is primarily in the length direction of the plate [8, and the degree to which this field is in the length direction depends upon the spacing of the bars H and I2. However, the voltage which is required to provide a given voltage gradient in the ceramic material varies with the spacing of the bars H and i2 so that, in practice, it has been found to be satisfactory if the spacing of the bars H and i2 is several times the thickness of the material to be polarized.

In order to polarize the plate in the length direction, the entire structure including the bars 1 i and i2 is slowly moved in the length direction of the material. Generally speaking, the degree of remanent polarization which is present in such a ceramic material is a function of the voltage as well as of the time the voltage is applied. Usually, in polarizing at normal room temperatures, it is desirable to provide a voltage gradient of around 75,000 volts per inch and to allow this voltage gradient to be. applied to the material for several minutes. Therefore,

the structure including bars H and i2 can be moved lengthwise along the plate ['0 at such aratethat each part of the material is subjected to such a voltage gradient for this period of time. Alternatively, the structure including bars I i and H can be moved at a faster rate and the operation can be repeated by bodily moving the structure back to the starting point but out of contact with the ceramic material to be polarized after the end of the material has been reached. In some cases, it is desirable to provide an arrangement which, continuously over a relatively long period of time, wipes the polarizing structure in the same direction across the surface of the titanatematerial. Various mechanical arrangements for eff'ectuating this type of action will be readily apparent to those skilled in the art.

It will also be apparent that, rather than utilizing a continuous movement of the polarizing structure including bars H and I2 in the length direction of the material, a step-by-step movement can be utilized. For example, the structure can be left in the position shown for an appreciable period of time and thereafter can be moved bodily to a position further along the material in the length direction. However, these steps should, in general, be no longer than the spacing between the bars It and i2 and, for most polarizing operations, the continuous movement described above is preferable. However, it will be understood that either of the methods described abve comprises a method of polarizing the plate l0 in a predetermined direction, specifically in the length direction. Also, either of the described methods includes the step of applying a unidirectional polarizing voltage between a first two points, say between points 2 and 25, on a surface of the material for an: appreciable period of time and of sufiicient magnitude as to effect a polarization in the material between these two points. The polarization here contemplated need not be the entire polarization ultimately effected in the material inassuch as the process may be repeated one or more times as mentioned above to increase the degree of polarization. Either or the two methods described above also comprises the step of subsequently applying a unidirectional polarizing voltage between a second two points, for example, points 25 and 21 on the surface. of the material. The points 2 6 and 21 are spaced from the points 24, 25 in the desired direction of polarization in the material, namely, in the length direction 01' the plate Ill. Also, one of the seci g and two points 26 and Z1 is between the first two points 24 and 25 and the other of the second two points, namely point 211, is spaced from the first two points 24 and 25 in thedesired direction of polarization, namely in the length direction. Also, either method described above, specifically the continuousv movement of the structure including bars H and I! or the step by-stcp movement thereof, involves an eiiecti ve moving in the length direction on the sur- The method includes the step of applying a polarizing voltage to an incremental length of one surface of this material and includes the further step of subsequently moving the points of application of the voltage on this surface in the direction of this incremental length.

Referring now to Fig. 2, the embodiment of the invention there illustrated is generally similar to that of Fig. 1 and corresponding elements have identical reference numerals. The Fig. 2 embodiment differs from that of Fig. 1 only in that the bars [I and II are not directly moved over the surface of the ceramic transducer material during the polarizing operation. In the Fig. 2 arrangement, conductive bristles or flexible strips 30, 30 are provided. These are attached to the bars H and I2 and effectively provide brushes which make a more intimate contact with the ceramic material under some circumstances than do the bars H and I2 of the Fig. l embodiment.

Reference is now made to Fig. 3 for the purpose of illustrating a modification of the Fig. 2 arrangement. In Fig. 3, the method of the invention is illustrated as being used for polarizing a ceramic transducer material in the form of a hollow right circular cylinder 3|. Elements of Fig. 3 which are generally similar to those of Fig. 2 have identical reference numerals. The operation of the Fig. 3 embodimen t will be readily understood from the-previous description. In the Fig. 3 arrangement, a polarizing voltage from the source 22 is applied to the ceramic material 3| through the brushes 30-, 3-8- and the brushes are moved in a circumferential direction along the surface of the cylindcr 3| as. indicated by the arrow 33. This rela tive motion may be effected by the movement of the entire structure including the bars II and 12' or may be eiiected by a rotation of the cylinder 3|. In .any event, the field pattern the cylinder is generally as represented by the dotted lines and is primarily in a circumferential direction in the material. The relative motion, as pointed out above, can be either so slow that the entire desired polarization is effected during one; rotation of the cylinder 31, or the cylinder 3| can be rotated at a higher speed. In the latter case, the voltage is maintained applied. for a sufficient time to effect the desired polarization in all parts of the material.

Reference is made to Fig. 4 for illustrating a modification of the Fig. 3 embodiment wherein a second set of brush electrodes is provided the olinder. Elements of Fig. 4 which are identical to those of Fig. 3 have identical reference numerals. Elements which are similar have identical reference numerals primed. Thus the brushstructure utilized inside the tube 3! of Fig. 4 includes conductive bars II and I2 having conductive bristles 3d, 36'. Only the insulating spacer member i5 is visible in the showing of Fig. 4, but it will be understood that the structure of the inside brush-electrode device is generally similar to that of the outside brush-electrode device. Furthermore, it will be seen that the inside brushes which directly oppose those on the outside have the same potential applied thereto so that there is no electric field directly through the material of the cylinder 3! in the radial direction thereof.

The operation of the Fig. 4 embodiment will be readily understood by those skilled in the art from the previous descriptions of operation. Here, however, the field pattern in the ceramic material is somewhat as illustrated by the dotted lines 36 and it will readily be seen that this field pattern is preferable to that illustrated by the numeral 34 of Fig. 3 where a thick material is to be polarized. It will furthermore be understood that the cylinder of Fig. 4 has been utilized in describing the operation of the double-brush arrangement simply for purposes of illustration and that a double-polarizing structure may be utilized in any of the other embodiments as well. In any case, this double-brush structure provides an arrangement in which a unidirectional polarizing voltage is applied between a first two points spaced in a given direction on a surface of the material, for example, between two points on the outside surface of the cylinder of Fig. 4. When the double-brush structure is utilized, there is simultaneously applied a corresponding unidirectional polarizing voltage between a first two points on the other of the surfaces of the cylinder, namely, on the inside surface of the cylinder of Fig. 4, the voltages applied inside and out being corresponding ones in that they produce a field in the same direction in the material. Specifically, the two points of voltage application on the inside surface oppose the two points of voltage application on the outside surface. Also, in an embodiment where the double-brush structure is used, there is subsequently applied to the ceramic material a unidirectional polarizing voltage between another two points on the one surface mentioned above, specifically the outside surface in the illustration of Fig. 4. These other two points are spaced from the first two points on the outside surface in the direction of desired polarization with one of these other two points being between the first two points, namely the firstmentioned two points on the outside surface of the surface of the cylinder, and the other of the two points on the outside surface being circumferentially spaced from the first two points on the outside surface under consideration in the predetermined direction of polarization. Also, in the embodiment under consideration, there is applied, simultaneously with the above-mentioned subsequent application of voltages to the outside surface of material, a corresponding unidirectional voltage between a predetermined other set of two points on the inside surface of the material which substantially oppose the abovementioned other two points on the one surface mentioned above, specifically the outside surface.

It will be understood that any of the embodiments of the invention mentioned above may be utilized in polarizing a ceramic transducer material at normal room temperatures, for example in air. However, it is also possible to utilize the method of the invention in polarizing at elevated temperatures, for example in oil. In such a case, it is generally preferable to place the entire structure in an oil bath which is heated to a temperature just below the Curie point of the ceramic material involved. The polarizing voltage is then applied to the material and all portions of the ceramic transducer material which are desired to be polarized are treated while the ceramic is maintained at the temperature near the Curie point. The temperature of the oil bath is next reduced by a small factor and the polarizing potentials are again applied in the same manner to all portions of the ceramic material desired to be polarized. The cylinders illustrated in Figs. 3 and 4 are specifically suitable for this arrangement in that they may be continuously rotated so that all portions of the ceramic material are subjected to the desired polarizing voltage before the temperature of the oil bath is materially reduced. Thereafter the temperature of the bath is further reduced and the process is continued until a temperature quite far removed from the Curie point of the material is reached.

By the invention, applicant has provided an arrangement whereby ceramic transducer material of very long length can be polarized using, for example, the arrangement of Fig. 1. In fact, there is no reason whatever why the method of the invention cannot be used to polarize a continuous strip of ceramic transducer material thereby effectuating a result which is impossible by any of the normal methods of polarization. It will be seen that the voltage gradients in the material, where applicants method is used, can be as high as where any other method of polarization is used. Furthermore, it will be seen that some types of polarization can be effectuated by the method of the invention which are impossible to effectuate by any other method generally known to those skilled in the art. Specifically, it is not possible by any of the normal methods to effectuate a circumferential polarization of a cylinder in the manner in which it is done by the methods of applicants invention described in connection with Figs. 3 and 4.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The method of polarizing in a predetermined direction a fired ceramic transducer material which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a unidirectional polarizing voltage between a first two points, spaced in said direction on a surface of said material, for an appreciable period of time and of sufficient magnitude as to effect a polarization in said material between said two points; and subsequently applying a unidirectional polarizing voltage between a second two points on said surface spaced from said first two points in said direction, with one of said second two points being between said first two points, and the other of said second two points being spaced from said first two points in said predetermined direction.

2. The method of polarizing in a predetermined direction a fired ceramic transducer material which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a unidirectional polarizing voltage between a first two points, spaced in said direction on a surface of said material, for an appreciable period of time and of sufficient magnitude as to effect a polarization in said material between said two points; and subsequently effectively moving in said direction on said surface the points of application of said voltage to said material.

3. The method of polarizing in a predetermined direction a fired ceramic transducer material which is capable of retaining a remanent polarization after having had a unidirectional voltage applied theret comprising: applying a unidirectional polarizing voltage between a first two points, spaced in said direction on a surface of said material, for an appreciable period of time and of sufiicient magnitude as to effect a polarization in said material between said two points; and subsequently moving in said direction on said surface the points of application of said voltage to said material to provide a succession of succeeding pairs of points of voltage application to said material such that one point of a particular pair is always between the points of the preceding pair, while the other of the points of said particular pair is always spaced from the point of the preceding pair in said predetermined direction.

4. The method of polarizing a fired ceramic transducer material, which has one relatively small dimension between two surfaces of said material, in a direction which is substantially normal to said dimension, the material being one which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a unidirectional polarizing voltage between a first two points, spaced in said direction on a surface of said material, for an appreciable period of time and of suflicient magnitude as to effect a polarization in said material between said two points; and subsequently applying a unidirectional polarizing voltage between a second two points on said surface spaced from said first two points in said direction, one of said second two points being between said first two points, and the other of said second two points being spaced from said first two points in said predetermined direction.

5. The method of polarizing a fired ceramic transducer material, which has one relatively small dimension between two surfaces of said material, in a direction which is substantially normal to said dimension and the material being one which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a unidirectional polarizing voltage between a first two points, spaced in said direction on one of said surfaces of said material, for an appreciable period of time and of sufficient magnitude as to effect a polarization in said material between said two points; simultaneously applying a corresponding unidirectional polarizing voltage between a first two points on the other of said surfaces which oppose the said first two points on said one surface; subsequently applying a unidirectional polarizing voltage between another two points on said one surface which are spaced from said first two points in said direction with one of said other two points being between said first two points on said one surface and the other of said other two points being spaced from said first two points on said one surface in said predetermined direction; and, simultaneously with said subsequent application of voltage to said one surface, applying a corresponding unidirectional polarizing voltage between a predetermined other set of two points on said other surface which substantially oppose said other two points on said one surface.

6. The method of polarizing a fired ceramic transducer material which has a relatively short dimension and a relatively long dimension substantially normal thereto and which material is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto which comprises: applying a unidirectional polarizing voltage between two lines on a surface or said material, each of which is substantially normal to said two dimensions, the lines being spaced in said longitudinal direction on said surface and the voltage being applied for a period of time and of sufficient magnitude as to effect a polarization in said material between said two lines; and subsequently simultaneously moving the lines of application of said potential to said surface in said longitudinal direction.

'7. The method of polarizing in a predetermined direction a fired ceramic transducer material, in a form having two relatively closely spaced surfaces which are substantially equidistant at all corresponding points thereof, and which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a polarizing voltage to an incremental length of a surface of said material; and subsequently moving the points of application of said voltage on said surface in the direction of said incremental length.

8. The method of polarizing in a predetermined direction a fired ceramic transducer material, of a form having two relatively closely spaced surfaces which are substantially equidistant at all corresponding points thereof, and which material is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a polarizing voltage to an incremental length of one of said surfaces; applying simultaneously a corresponding polarizing voltage to a corresponding incremental length of the other of said surfaces; subsequentl moving the points of application of said voltage on said one surface in the direction of said incremental length; and, simultaneously with the moving of the points of application of said voltage on said one surface, moving the points of application of said voltage on said other surface correspondingly in the direction of said incremental length.

9. The method of polarizing in a circumferential direction a hollow cylinder of fired ceramic transducer material which is capable of retaining a remanent polarization after having had a unidirectional voltage applied thereto comprising: applying a unidirectional polarizing voltage between two points on a surface of said cylinder, which are spaced in a circumferential direction on said surface, for an appreciable period of time and of sufficient magnitude as toeffect a polarization in said material between said two points; and subsequently simultaneously moving the points of application of said voltage along the surface of said cylinder in a circumferential direction to effect a polarization in other circumferential portions of said cylinder.

ALFRED L. W, WILLIAMS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,521,329 Begun Sept. 5, 1950 2,532,100 Howell Nov. 28, 1950 2,540,194 Ellett Feb. 6, 1951 2,540,412 Adler Feb. 6, 1951 

